The present invention relates to a device and a method for forming a heat seal between two layers of sheet material. More specifically, the invention relates to the welding of a trailing sheet tail on a roll of sheet material to an underlying layer of sheet material on the roll to form a finished roll product ready for packaging.
Sheet material converted from base rolls into roll product in winder lines generally is wound to a selected diameter and then cut to form a sheet material roll. One of the problems inherent in the production of sheet material rolls is that the sheet web, once cut, forms a trailing sheet tail which has to be bonded to an underlying layer of sheet material to form a finished roll product ready for packaging. If the trailing sheet tail is not properly bonded to the roll, it becomes unfurled or folded over, and may even unwind to present a loose end. Apart from interfering with in-plant handling of the roll, this leads to problems with wrapping and can result in machine downtime. Furthermore, where a clear poly-wrap is used to wrap the roll, the defects are visible through the wrapping, and this detracts from the appearance of the finished product.
Attempts have been made to fasten the tails of sheet material rolls with glue or a similar bonding agent. For example, U.S. Pat. No. 3,044,532 to Ghisoni discloses a device in which a roll of sheet material carried on a conveyor is unwound slightly to expose the free end of the tail, and after application of a glue to the underside of the free end, is rewound to adhere the tail to the roll. In U.S. Pat. No. 3,172,612 to Besserdich, thermoplastic adhesive is applied in a pattern to the underside of the free end of the roll, the free end is rolled against the body of the roll, and a heated, rotating press roll is pressed against the outside of the free end to make the thermoplastic tacky and secure the free end to the roll. Although the use of glue or a similar bonding agent is suitable for certain applications, glue can stain sheet material imparting an unattractive appearance to the roll. Furthermore, glue tends to soak through multiple layers of the sheet material on a roll, and unless the glue is self-releasing and effective for only a limited time after application, the roll is difficult to unwind without tearing and wasting part of the roll. Also, where various different types of sheet materials are converted from base rolls into roll product on the same winder line, a glue which is suitable for some of the sheet materials may not be suitable for others, and often it is not cost-effective to employ two or more glue systems, or to utilize a grade of glue that is effective on all of the sheet materials.
In the art of packaging, it is known to use heat seals for bonding together overlapping sheets of plastic film. One apparatus for producing such a heat seal is disclosed in U.S. Pat. No. 5,935,379 to Frantz et al. This apparatus includes a plurality of rotatable sealing wheels and bearing disks which co-operate with one another on opposed sides of two overlapping edges of a plastic film to form heat seals in the film. This type of heat sealer requires access to the layers of plastic film from opposed sides thereof, and therefore is limited to applications where film layers are accessible from both sides.
U.S. Pat. No. 5,791,125 to Kallner discloses a rotary heat sealer for thermally sealing overlapping film layers wrapped about a load by engaging the outer surface of the outer film layer. The heat sealer in this disclosure includes heat sealing disks which are resiliently mounted to a rotatable shaft and which have peripherally disposed heating elements for engaging overlapping film layers wrapped about the load to thermally seal the film layers together. The heat sealing disks are hollow and contain coil springs for effecting the resilient mounting of the disks to the rotatable shaft. Although this apparatus allows for bonding from one side only of the overlapping film layers, it includes numerous design components and is relatively complex in construction. Furthermore, since the heating elements are positioned on the periphery of the disks, the capacity of the disks to store heat energy is limited, and hence this apparatus is restricted to relatively slow-speed wrapping applications. Apart from this, the physical dimensions and characteristics of commercially available heaters for performing the heat seal limit the bond width and the types of bond patterns achievable.
There remains a need in the art for a device and a method for effecting a heat seal between the trailing sheet tail of a roll of sheet material and an underlying layer of sheet material on the roll by engaging a rotary heat sealing disk with the outer surface of the trailing sheet tail.
What is also needed is a heat sealing device having a rotary heat sealing disk which can store sufficient heat energy to sustain continuous sealing of the trailing sheet tail to the roll of sheet material, in-line with product flow, without affecting continuous production speeds.
There is also a need in the art for a device for bonding various different types of sheet material tails to underlying layers of sheet material on the same winder line.
Furthermore, there remains a need for a heat sealing device which exhibits flexibility with regard to the bond width of the heat seal and/or the types of bond patterns achievable on the heat seal.